In order to increase the number of subscribers that can be serviced in a single wireless network, frequency reuse is maximized by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. Accordingly, the greater number of base transceiver stations increases infrastructure costs. To offset this increased cost, wireless service providers continually implement any improvements that may reduce equipment costs, maintenance and repair costs, and operating costs, or that may increase service quality and reliability, and the number of subscribers that the cellular system can service.
In many receivers characterization of forward path gain and calibration of the received signal strength indicator (RSSI) signal are presently accomplished with temperature compensation circuitry. The temperature compensation circuitry adapts to variations in gain, attenuation, and detector slopes over a range of temperatures and frequencies. Because the characteristics of devices and components used in the temperature compensation circuitry change with variations in temperature and frequency, the receivers must be calibrated and characterized at the time of manufacture. However, the characteristics of the devices and components vary within different manufacturing lots. This means that the operating characteristics of the receivers must be continuously monitored during the manufacturing process to detect changes that occur as the manufacturing process progresses.
Therefore, the receiver circuitry must be characterized by analyzing numerous individual receiver units during the manufacturing process in order to develop an accurate profile for the temperature compensation circuitry. After the receiver circuitry has been characterized, the characterization information must be stored in the memory of each of the individual receiver units. Because the manufacturing process produces component changes over a period of time, the receiver characterization process must be re-performed and the information in the memory of each of the individual receiver units must be updated.
There is therefore a need in the art for a receiver design that does not require continual re-characterization of forward path gain and continual recalibration of Received Signal Strength Indicator (RSSI) during the manufacturing process.
After a base transceiver station (BTS) has been manufactured, wireless service providers use a variety of test equipment to monitor the performance of the RF receiver and the RF transmitter in the BTS during operation. The test equipment may monitor a variety of signal parameters in the RF transmitter, including adjacent channel power ratio (ACPR), spectral purity (including in-band and out-of-band spurious components), occupied bandwidth, RHO, frequency error, and code domain power. The test equipment may also perform a variety of test functions in the RF receiver, including testing and measuring the receive antenna return loss and calibrating the receiver. Preferably, the signal parameters are remotely monitored from a central location, so that a wireless service provider can avoid the expense of sending maintenance crews into the field to test each BTS individually. Additionally, a remote monitoring system can detect the failure of an RF transmitter or an RF receiver nearly instantaneously.
Unfortunately, adding some types of test equipment (e.g., spectrum analyzers) to a BTS significantly increases the cost of the BTS. In some cases, the cost of the test equipment may be greater than the cost of the BTS itself. As a result, wireless service providers may not install any test equipment in the ETS. Alternatively, wireless service providers may install only a limited amount of test equipment to test only some of the functions of the BTS. The remaining functions must be monitored by maintenance crews using portable test equipment.
There is therefore a need in the art for inexpensive test equipment that may be implemented as part of the base station. In particular, there is a need for integrated test equipment that can reuse some of the existing circuitry in a base transceiver station. More particularly, there is a need for integrated test equipment that can be used to measure the impedance match of a receive antenna and that can be used to calibrate the receiver gain.
Prior art RF test injection circuits have been used to measure RF signals in an RF receiver in a base station in a wireless network for the purpose of measuring the impedance match of a receive antenna and to calibrate the receiver gain. A prior art injection circuit usually comprises a directional coupler that has an input coupled to a duplexer that is coupled to an antenna array. The output of the directional coupler is coupled to a signal amplifier. Also coupled to the directional coupler is an injection source that is capable of injecting a test RF signal into the directional coupler.
When a prior art injection circuit of this type is used to measure the impedance match of a receive antenna, the injection source injects a test RF signal into the directional coupler in the direction of the signal amplifier. Level detector circuitry that is coupled to the signal amplifier measures the RSSI level of the test RF signal to obtain a first RSSI measurement of the test RF signal.
Then the injection source injects a test RF signal into the directional coupler in the direction of the duplexer that is coupled to the antenna array. The test RF signal passes through the duplexer and hits the antenna array. RF signal energy that is not absorbed by the antenna array is reflected back through the duplexer and through the directional coupler to the signal amplifier and the level detector circuitry. The level detector circuitry coupled to the signal amplifier measures the RSSI level of the test RF signal to obtain a second RSSI measurement of the reflected test RF signal. The level detector circuitry compares the two RSSI measurements to obtain a voltage standing wave ratio (VSWR) that measures the impedance match of the antenna array.
One of the primary deficiencies of this prior art approach is the difficulty of controlling the directivity of the directional coupler. This is because directional couplers are, capable of providing only approximately 10 dB to 15 dB of reverse isolation between its input signal and its output signal. As a result, the directional coupler may transfer a signal that is 10 dB to 15 dB below its output signal back through the duplexer to the antenna array. The relatively low level of reverse isolation that is provided by the directional coupler means that a portion of the signal energy at the output of the directional coupler will be transferred back through the duplexer to the antenna array and reflected back through the duplexer to the directional coupler. The reflected energy adversely affects the RSSI measurements and causes an erroneous determination of the voltage standing wave ratio (VSWR). The same problem occurs when such a prior art injection circuit is used to calibrate the receiver gain.
There is therefore a need in the art for an improved test injection circuit for measuring radio frequency (RF) signals in an RF receiver.